Somewhere in the highlands of Afghanistan, a hungry fox pounces on a tasty-looking leopard gecko. But the lizard has a get-out-of-jail-free card: a detachable tail. The dropped appendage flails around long enough to distract the fox, allowing the gecko itself to run off and hide.

Leopard geckos are one of a few lizard species that possess this self-amputation ability, known as autotomy. The technique is effective, but the tail can account for about a quarter of the lizard's body mass. So how do these animals adapt to losing so much of it that quickly?

When geckos lose their tail, they “take this more sprawled posture” and walk with their limbs splayed out farther from their body, says Chapman University biologist Kevin Jagnandan. Most researchers initially assumed this stance was a response to a suddenly shifted center of mass. But when Jagnandan observed leopard geckos with a tail in his laboratory, he realized that they wag it as they walk, suggesting that these movements may be key to the lizards' locomotion.

To test this hypothesis, Jagnandan and his team assessed the postures of 10 geckos walking in various conditions: with their tail intact; with their tail restrained by a small section of glued-on fishing rod (of negligible mass); and with their tail self-amputated. These comparisons allowed the researchers to distinguish the effects of lost mass from those of lost tail-wagging on the geckos' movements.

The lizards with an immobilized tail adopted stances similar to those with no tail, the researchers reported in a study published in September in Scientific Reports. This result suggests the sprawling walk they adopt after losing their tail is not compensating for the missing mass but rather for the lack of tail-wagging. Jagnandan thinks tail movements help the lizards maintain balance and stability as they walk. He suspects that the tails of arboreal mammals, such as cats and monkeys, serve a similar purpose.

Bill Ryerson, a biologist at Saint Anselm College, who was not involved in the study, was surprised by the findings. “We thought we had settled it—it seemed pretty open and shut” that mass was the main factor, he says. The new study challenges this earlier idea in a “beautifully simple” way, Ryerson adds.

Jagnandan hopes that understanding how animals react to missing body parts could ultimately help engineers design robots that can move more efficiently as heavy loads—or even entire limbs—are added and removed.

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